UNITED NATIONS — Russia’s U.N. ambassador said Tuesday that Russian experts determined that Syrian rebels made sarin nerve gas and used it in a deadly attack outside Aleppo in March.

International analysts say a chemical weapon attack occurred March 19 in the government-controlled Aleppo suburb of Khan al-Assal. Ambassador Vitaly Churkin blamed opposition fighters for the attack, which he said killed 26 people, including 16 military personnel, and injured 86 others. The rebels have blamed the government for the attack.

Churkin told reporters after delivering an 80-page report to Secretary-General Ban Ki-moon that the Assad regime asked Russia, its closest ally, to investigate the attack after a U.N. team of chemical weapons experts was unable to enter the country in a dispute over the probe’s scope.

The samples taken from the impact site of the gas-laden projectile were analyzed at a Russian laboratory certified by the Organization for the Prohibition of Chemical Weapons, Churkin said.

The ambassador said the results of the analysis indicate the Bashar 3 rocket “was not industrially manufactured and was filled with sarin.” He said the samples indicated the sarin and the projectile were produced in “cottage industry” conditions.

The absence of chemical stabilizers, which are needed for long-term storage and later use, indicated its “possibly recent production,” Churkin said.

“Therefore, there is every reason to believe that it was the armed opposition fighters who used the chemical weapons in Khan al-Assal,” Churkin said.

“According to information at our disposal,” he said, “the production of Bashar 3 unguided projectiles was started in February 2013 by the so-called Bashar al Nasser Brigade affiliated with the Free Syrian Army.”

On Monday Syria invited Ake Sellstrom, head of the U.N. fact-finding mission on allegations of chemical weapons use in Syria, and U.N. disarmament chief Angela Kane to visit Damascus for foreign-minister level talks on conducting a probe of the Khan al-Assal attack. The Russian ambassador strongly backed the idea.

Britain, France and the United States have provided the secretary-general with information on other alleged chemical weapons attacks in Syria. Ban has repeatedly said he wants a broader investigation than just Khan al-Assal.

“We support a thorough investigation of all credible allegations,” Churkin said, but added that Russian experts were not convinced by the material provided to them by the U.K., U.S. and France.

President Barack Obama’s government says it has “high confidence” that Syrian President Bashar Assad’s forces have killed up to 150 people with sarin gas. The use of a chemical weapon crosses Obama’s “red line” for escalating U.S. involvement in the conflict and prompted the decision to send arms and ammunition to the opposition, not just humanitarian aid and non-lethal material like armored vests and night goggles.